In a first aspect, the present invention relates to an electronic thermometer and more particularly, but not exclusively, to a thermometer useful for measuring body temperature of human or animal subjects or for measuring ambient temperature. In a second aspect, the present invention relates to a telemetry system consisting of a transmitting measurement device (preferably a body temperature thermometer) and a portable receiving unit which displays the measured value of a sensed parameter.
Several prior art inventions utilize a temperature-dependent resistive element together with an oscillator circuit to form a digital thermometer. In the prior art, a thermistor is sometimes used as a temperature-dependent, variable-resistance device in series with a charging capacitor to form the frequency-controlling elements of the oscillator network. The equation   f  =      1          (              2        ⁢        π        ⁢                  xe2x80x83                ⁢        RC            )      
determines the frequency of oscillation, where R is the resistance of the resistive element (thermistor) and C is the capacitance of the series charging capacitor. As the temperature varies, the resistance of the thermistor varies, and the frequency varies as a result. By measuring the frequency, and knowing the value of the capacitance, the value of R can be determined. Because R is uniquely related to temperature, the temperature can be determined as well. For a thermistor, the resistance is related to the temperature via the Steinhart-Hart equation. The use of a multivibrator as the oscillator circuit is disclosed in U.S. Pat. No. 4,359,285 by Washburn for low-power oceanographic applications. U.S. Pat. Nos. 4,602,871 and 4,464,067 issued to Hanaoka disclose thermometers based on thermistor-controlled oscillators whose properties emphasize miniaturization, light weight, and improved accuracy using correcting circuits. These latter two patents refer to applications wherein the sensor may be used with low-power wristwatch devices.
One disadvantage of measuring the frequency of the oscillator is that one must know the value of the capacitor extremely accurately in order to derive the value of the resistance accurately. Generally, it is difficult to do capacitance measurements accurately, and in addition, the capacitance value is known to be a temperature-dependent parameter. The capacitance can increase or decrease with changing temperature and the degree of change is related to the exact type of material used in the capacitor (Y5V, X7R, NPO, etc.). A further disadvantage of this approach is that the active circuit elements in the oscillator circuit can themselves have temperature-dependencies. These dependencies are nearly impossible to predict and may vary from circuit to circuit.
Some prior arts attempt to reduce the undesirable temperature dependencies by way of calibration techniques. As an example of prior art, U.S. Pat. No. 4,150,573 discloses the use of a thermistor to control a pulse oscillator circuit. In that patent, the pulse oscillator input is switched between the thermistor and a fixed resistor. A ratio is formed between the frequency produced by the thermistor and the frequency produced by the fixed resistor. This ratio divides out uncertainties associated with circuit component values and power supply variations. This provides the advantage of reducing the need for high accuracy parts and reduces the effects of power supply variations. However, this concept is unnecessarily complicated and it does not accurately measure the non-ideal behavior of the oscillator circuit nor does it null out temperature dependencies in the active components of the oscillator circuit. This concept may also introduce errors due to the temperature variations in the switching device.
For a medical thermometer, or other applications where extreme accuracy is required (less than 0.05 degrees C. uncertainty), the errors introduced by capacitance variation and by active circuit element variation cannot be tolerated. A method is needed that reduces these effects to a level of less than 0.01 degrees C. In addition, for a low-power application such as a miniature ingestible temperature sensor, it is not possible to use sophisticated, computer-controlled correction techniques, because the thermometer must be miniature, and is expected to be powered from a 1.5 volt battery source or a 3.0 volt battery source.
At present there are ingestible temperature responsive transmitters or ingestible temperature monitoring pills available. U.S. Pat. No. 4,689,621 issued to Kleinberg, and U.S. Pat. No. 4,844,076 issued to Lesho et al describe temperature responsive transmitters for use in ingestible capsules. Both devices disclosed employ crystal-controlled oscillators which transmit continuously on a single frequency determined by the temperature of the device. Lesho et al. also discloses a receiver employing a frequency counter to determine the frequency of the transmitter, and perform the calculation to determine the temperature sensed by the pill.
However, both of these devices have severe application limitations as they are purely analog devices, continuously transmitting on a single frequency. This prevents the use of multiple devices on a single subject, or on subjects in close proximity to each other because the signals from individual devices interfere with each other and cannot be distinguished. In addition, the prior art uses the temperature characteristics of a crystal to vary the oscillation frequency of the transmitter, requiring a frequency counter or other coherent detector in the receiver to determine the absolute frequency, and hence, the temperature. Use of a crystal to determine the oscillation frequency also requires an extensive calibration procedure, and requires the user of the device to input those calibration values into the receiver prior to use.
To prevent the batteries used in the ingestible capsule from being drained during storage, the prior art places a magnetic reed switch between the battery and the circuitry. Consequently, the device must be stored with a magnet in close proximity to keep the device de-activated, or it must use a rechargeable battery, and a recharger as disclosed in Lesho et al.
The basis of the invention in a first aspect is a circuit containing a temperature-dependent resistive element that controls the charge and discharge times of a multivibrator. By measuring the charge and discharge times, and converting those time elements with a formula, the resistance value of the resistive element can be determined. Because the resistance of the resistive element is uniquely related to temperature, the temperature can be uniquely determined.
As in the prior art, our invention also utilizes a thermistor-controlled multivibrator whose oscillation frequency is determined by the RC combination of the thermistor resistance and the value of the charging capacitor. However, the preferred embodiment contains several unique designs not introduced in the prior art. These designs provide novel means to (1) null out errors introduced by the non-ideal behavior of the multivibrator circuit and (2) vastly improve accuracy by nulling out undesirable temperature-induced effects within the passive and active circuit elements.
Our invention utilizes a CMOS 555 timer as the multivibrator circuit in the preferred embodiment. However, other oscillator designs might be used in other applications as well. For example, our method could be used with a bipolar, 5-volt 555 timer, when higher voltage power supplies would be available.
A first novel feature of the preferred embodiment of the first aspect of this invention is the determination of temperature through the measurement of the charge and discharge times of the sensor digital waveform. By measuring the ratio of the discharge time to the charge time, a sensor response may be obtained that is uniquely determined by the temperature that the sensor is in equilibrium with, e.g. body temperature, skin temperature, ambient temperature, etc.
A second novel feature of the preferred embodiment of the first aspect of this invention is to measure the cell constant of the sensor by substituting a precision fixed reference resistor in place of the temperature-dependent resistor, and measuring the response of the sensor when the fixed resistor is in place. All subsequent measurements of the sensor response when the temperature-dependent resistor is in place are normalized by the cell constant, thereby nulling out non-ideal effects of the astable multivibrator and the passive circuit elements that control the multivibrator frequency.
A third novel feature of the preferred embodiment of the first aspect of this invention is that the fixed reference resistor is a precise multiple value of the temperature-dependent resistor when the temperature-dependent resistor is stabilized at a characteristic temperature. For a thermistor, this characteristic temperature is known as the xe2x80x9creference temperaturexe2x80x9d and the resistance of the thermistor at the characteristic temperature is known as the reference resistance. Ideally, the multiple value is a positive integer and is preferably unity.
A fourth novel feature of the preferred embodiment of the first aspect of this invention is that the cell constant may be measured during assembly of the sensor by substitution of the reference resistor of known value instead of the temperature-dependent resistor and this process does not require the use of a temperature-stabilized immersion bath or chamber.
In accordance with the second aspect of the invention, a body temperature measurement system consists of a microcontroller-based measurement device which transmits body temperature data, and a body-worn receiving unit which interprets the transmission and displays the temperature.
The measurement device contains three electrical subsystems: a thermistor-controlled multivibrator sensor circuit, a low-power microcontroller, and a modulated transmitter. A precision thermistor provides a variable charge element for an integrated timer circuit connected in an astable multivibrator configuration. The multivibrator generates pulses whose duty cycle varies as the thermistor charge element varies. The pulses are counted by a low-power microcontroller, and converted by the same microcontroller to a digital number which is temporarily stored in memory. Once the digital number has been determined the microcontroller de-activates power to the multivibrator to conserve battery power.
Using its own clock and a seed value, the microcontroller calculates a pseudo-random number, and determines the commencement of the next data transmission. The microcontroller then constructs a data word consisting of the current reference clock value, the digital number from the multivibrator count, a unique serial number previously stored in the microcontroller during manufacturing, and an error detection number calculated from the other values in the data word. The data word is interleaved with very specific bit values, and attached to a preamble and sync word to create a data packet. When it is time for the next transmission, the microcontroller activates the RF transmitter, and begins sending the data packet to the modulation portion of the RF transmitter.
The RF transmitter consists of a crystal-stabilized oscillator which supplies a carrier frequency, and a variable capacitor which frequency modulates the carrier. Once it has been activated, the RF transmitter modulates the data packet from the microcontroller onto the carrier, and transmits the modulated carrier. After the data has been transmitted, the microcontroller de-activates the RF transmitter, and enters a sleep mode to preserve battery power. The microcontroller leaves sleep mode just prior to the next transmission, re-actives the multivibrator, and begins the temperature measurement cycle again.
The entire measurement device electronics are designed to operate from one or two silver-oxide batteries. The device and the batteries are packaged in a bio-compatible shell that can be swallowed.
The receiving unit contains three functional subsystems: data demodulator and interpreter, microcontroller with sensor-tracking and data conversion algorithms, and activation mechanism. The data demodulator consists of a radio which demodulates the carrier to data. An on-board microcontroller removes the interleaved bits and checks the error detection word to determine if the incoming data is uncorrupted by the RF channel. For intact data words, the microcontroller converts the digital number from the multivibrator count to temperature. The temperature and the serial number from the data word are displayed on an LCD, stored in on-board memory, and/or retransmitted to a remote station via a direct-wire connection or a secondary radio-frequency link.
The same pseudo-random algorithm used to determine the next transmission time in the transmitter is programmed into the receiving unit microcontroller. Using this algorithm, the receiver can predict when each sensor will transmit a data packet, and which sensor will be transmitting the data packet. If the receiving unit is displaced far enough from the measurement device that radio communication is lost, the receiver can still predict when that particular transmitter should be transmitting based on the algorithm.
For the pseudo-random algorithm to be initiated correctly, the receiving unit has an embedded activation mechanism. A cavity within the receiver""s housing holds the measurement device capsule. The receiver""s microcontroller pulses an IR LED contained within the cavity. An IR sensitive photodetector in the device capsule activates the capsule""s microcontroller. Subsequent messaging from the receiver via the IR LED confirms the capsule""s use and prompts a transmission from the measurement device. This first transmission from the measurement device contains the device""s serial number, clock value, and a calibration value stored during manufacturing. The serial number identifies the device during all subsequent transmissions. Included in the serial number is a sensor identifier that indicates which type of physiological parameter the sensor measures. Feedback is provided to the user to indicate that the sensor has been identified and is operational. The temperature monitor system is able to support sensors of other physiological parameters (including but not limited to heart rate, blood pressure, SPO2, etc.), behavioral parameters (such as activity, sleep, etc.), environmental conditions (such as temperature, motion, etc.), and detection conditions (such as concentration level of a toxic or other material of interest).
The clock value is used as part of the sensor-tracking algorithm, and the calibration value is used to convert the digital number from the multivibrator to temperature.
Using the on-board activation mechanism, several devices can be monitored by the same receiving unit, since each one has a unique serial number, and is given a different time sequence in the sensor-tracking algorithm.
A first novel feature of the preferred embodiment of the second aspect of this invention is the use of a multivibrator to convert the thermistor resistance to pulses with varying duty cycle, counting these pulses by an onboard microcontroller and storing a resulting value in memory as a digital number.
A second novel feature of the preferred embodiment of the second aspect of this invention is that a unique serial number is stored in the device microcontroller memory and is transmitted with the measured temperature number to specifically identify the device. The serial number contains a sensor identifier indicating which physiological (heart rate, blood pressure, temperature, etc.), behavioral, environmental, or detection parameter is being measured.
A third novel feature of the preferred embodiment of the second aspect of this invention is that the sensor/transmitter microcontroller determines when the next data transmission will be by calculating a pseudo-random number based on its own clock and a seed number. The same algorithm is contained in the receiving unit so that the receiver can predict when the next transmission will be from each device.
A fourth novel feature of the preferred embodiment of the second aspect of this invention is that the measured temperature number, the clock cycle, and the unique serial number are combined in a data word with an error detection number. This data word is encoded using bit stuffing, and combined with a preamble and sync word to form a data packet.
A fifth novel feature of the preferred embodiment of the second aspect of this invention is that the RF transmitter topology includes a mechanism for modulating the carrier. Further, the RF transmissions are periodic, rather than continuous, and are not started until the time calculated by the microcontroller.
A sixth novel feature of the preferred embodiment of the second aspect of this invention is that the microcontroller controls the activation of the multivibrator and the RF transmitter, and utilizes a sleep mode to minimize the average current draw on the battery. This allows the use of ultra-miniature silver-oxide batteries.
A seventh novel feature of the preferred embodiment of the second aspect of this invention is an activation mechanism utilizing an IR LED and an IR sensitive photodetector. The mechanism activates the microcontroller reset in the measurement device allowing it to be shelved for long periods before use. The IR communication link also enables the receiving unit to send instructions to the device, and prompt the device for information such as the serial number and calibration value. The IR communication link also enables the receiving unit to determine whether the sensor is operating within limits so that feedback may be provided to the user about the satisfactory operation of the sensor.